In prior art high-head multistage pump-turbines, the rotor is mounted on two hydrodynamic bearings respectively located in the upper part and the lower part. The lower bearing is mounted on the suction bend of the turbine, a labyrinth seal being located above the latter for creating a seal between the shaft and the suction bend. This labyrinth seal involves a significant addition of the axial space requirement, resulting in a fairly significant center-to-center distance between the two bearings. During the runs in transient operation, considerable radial thrusts appear at the wheels, giving rise, because of this significant center-to-center distance, to distortions in the line of shafts. As a result, it is necessary to provide, in the region of the labyrinth seals fitted to the wheels, substantial clearances, in turn causing substantial losses through leakage and hence loss of output.
Moreover, this increase in the dynamic deflections, in both normal and transient operation, constitutes a factor in the increase of wear of the various labyrinth seals, as well as an increase of noise. To reduce this dynamic deflection it would be necessary to raise the lower hydrodynamic bearing, but this would entail an increase in the angle of the internal cone at the bend, and thus a bulkier external profile of this bend for an equal cross section of flow. Not being optimal, this new bend profile would give rise to phenomena of cavitation with attendant wear, as well as a loss of output.